From c4ebf2571a497fa0889dd9217c1d3616b2160cc9 Mon Sep 17 00:00:00 2001
From: VicRen <vic.ren@me.com>
Date: Tue, 27 Oct 2020 09:51:19 +0800
Subject: [PATCH] 27: quadratic primes

---
 27_quadratic_primes/main.go      | 50 ++++++++++++++++++++++++++
 27_quadratic_primes/main_test.go | 62 ++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)
 create mode 100644 27_quadratic_primes/main.go
 create mode 100644 27_quadratic_primes/main_test.go

diff --git a/27_quadratic_primes/main.go b/27_quadratic_primes/main.go
new file mode 100644
index 0000000..f423df3
--- /dev/null
+++ b/27_quadratic_primes/main.go
@@ -0,0 +1,50 @@
+package main
+
+import "fmt"
+
+func main() {
+	maxN := 0
+	maxA := 0
+	maxB := 0
+
+	cm := 0
+	for a := -999; a < 1000; a += 2 {
+		for b := 3; b < 1000; b += 2 {
+			fmt.Println("a:", a, "b:", b)
+			cm = findMaxN(a, b)
+			if cm > maxN {
+				maxN = cm
+				maxA = a
+				maxB = b
+			}
+		}
+	}
+
+	fmt.Println("product is:", maxA*maxB)
+}
+
+func findMaxN(a, b int) int {
+	max := 0
+	n := 2
+	for isPrime(n*n + a*n + b) {
+		if n > max {
+			max = n
+		}
+		n++
+	}
+	return max
+}
+
+func isPrime(n int) bool {
+	if n <= 1 {
+		return false
+	}
+	divider := 2
+	for divider < n {
+		if n%divider == 0 {
+			return false
+		}
+		divider++
+	}
+	return true
+}
diff --git a/27_quadratic_primes/main_test.go b/27_quadratic_primes/main_test.go
new file mode 100644
index 0000000..892d6d3
--- /dev/null
+++ b/27_quadratic_primes/main_test.go
@@ -0,0 +1,62 @@
+package main
+
+import "testing"
+
+func Test_isPrime(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tt := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			"1",
+			args{1},
+			false,
+		},
+		{
+			"2",
+			args{2},
+			true,
+		},
+		{
+			"4",
+			args{4},
+			false,
+		},
+		{
+			"17",
+			args{17},
+			true,
+		},
+		{
+			"37",
+			args{37},
+			true,
+		},
+		{
+			"73",
+			args{73},
+			true,
+		},
+		{
+			"13 * 3 * 5 * 5 * 17 * 31 * 737",
+			args{3 * 3 * 5 * 5 * 17 * 31 * 73},
+			false,
+		},
+		{
+			"large_one",
+			args{2876*2876 - 63*2876 + 977},
+			false,
+		},
+	}
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := isPrime(tc.args.n); got != tc.want {
+				t.Errorf("isPrime() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}